Example use case

[FrancisBanville]

I added an example use case in the documentation, following suggestions of #171.

Pinging @tpoisot

[tpoisot]

You should be using the syntax from Documenter for these examples - have a look at https://raw.githubusercontent.com/EcoJulia/SimpleSDMLayers.jl/master/docs/src/examples/gbif.md

This will make the examples run, and produce the figures automatically -- also check the Mangal documentation so the use-case is not duplicated. Another option is to have one of the examples from here as doc examples.

[FrancisBanville]

Thanks @tpoisot.

I made some changes using the appropriate syntax, and updated the example so to avoid too many duplications with the Mangal documentation. What do you think?